This invention relates to a gas absorption system for purifying a gas flow contaminated at least by carbon dioxide and hydrogen sulfide, utilizing a sour gas scrubbing agent selective for hydrogen sulfide, from which an off-gas enriched in hydrogen sulfide is removed and fed fo a sulfur recovery means.
A process step frequently required in the processing of raw gas streams is the separation of sour gases which are understood to encompass essentially carbon dioxide, hydrogen sulfide, carbonyl sulfide, hydrocyanic acid, and mercaptans. For one or more reasons, e.g., corrosion or catalyst poisoning, such acidic compounds must be removed prior to downstream treatment of the residual gaseous components. Most frequently found and generally in the highest concentrations of the raw gas streams are carbon dioxide and hydrogen sulfide.
Examples of such raw gas streams include, but are not limited to, natural gas, cracked gases, and especially hydrogen-containing gaseous mixtures. To produce industrially useful hydrogen-rich gaseous mixtures, for example, feed gas for hydrogenations, ammonia synthesis, methanol synthesis, etc., conventional starting materials at the present time are crude oils, refinery residue oils, coal, natural gas or similar carbon-containing substances. These raw materials, which in most cases contain sulfur, are subjected to oxidative thermal cracking with oxygen (reforming) at an elevated temperature. After the separation of entrained solids and liquids, e.g., soot, tar, naphthalenes, higher hydrocarbons, and water, a gaseous mixture is obtained consisting essentially of hydrogen, carbon oxides, and hydrogen sulfide, with possible traces of nitrogen, argon, methane and other impurities. If it is intended to use this gas for oxo synthesis, the sour gases are immediately removed so as to provide a synthesis gas consisting essentially of carbon monoxide and hydrogen. In contrast, if it is intended to obtain a gas consisting essentially only of hydrogen, for example hydrogenation hydrogen, or a feed gas mixture for ammonia synthesis, then the carbon monoxide contained in the gas is subjected to a water-gas shift conversion, resulting in oxidation of carbon monoxide to carbon dioxide and in the formation of additional hydrogen. In such a shift conversion, any mercaptans and any carbonyl sulfide which may be contained in the raw gas are reduced to hydrogen sulfide so that the sour gas to be removed from these gases consists essentially of only carbon dioxide and hydrogen sulfide.
Conventionally, the off-gas from the sour gas removal system comprises CO.sub.2 and H.sub.2 S. Whereas CO.sub.2 is ecologically acceptable except for the controversial greenhouse effect, the H.sub.2 S, owing to its high toxicity, cannot be exhausted into the environment except in exceedingly low legally acceptable concentrations. Consequently, H.sub.2 S is generally converted into elementary sulfur in a sulfur producing plant, e.g., a Claus plant. Because the ecologically harmless carbon dioxide exhibits no significant, beneficial effect in a sulfur production plant and conversely demands larger expenditures for equipment and energy, selective scrubbing of hydrogen sulfide is employed on a large scale to separate H.sub.2 S from CO.sub.2 in the sour gas scrubbing system. In such methods, when the scrubbing agent is regenerated, there is obtained, in addition to a gas stream rich in hydrogen sulfide and containing only part of the separated carbon dioxide, also a residual gas stream containing carbon dioxide and essentially free of hydrogen sulfide. This residual gas stream can be either directly discharged into the atmosphere, or if desired, pure carbon dioxide may also be recovered and used for conventional purposes.
Both chemical and physical scrubbing methods have been developed for the selective separation procedure. For many years now, the physical scrubbing methods have been preferred for large scale usage, especially to purify a gas stream relatively strongly contaminated with carbon dioxide. The physical absorption or scrubbing liquids are used therein to dissolve the sour gas components without simultaneous chemical reaction and can be liberated to these components by expansion, heating and/or distillation. For the separation of carbon dioxide and hydrogen sulfide, in particular, it has been found suitable to use polar organic solvents, especially methanol which is capable of dissolving large amounts of sour gas below 0.degree. C.
The off-gas fraction enriched with hydrogen sulfide to an extent dependent on the hydrogen sulfide content of the raw gas to be purified ordinarily contains between 10 and 70% by mole of hydrogen sulfide and is processed for instance in a sulfur recovery plant based on the Claus-reaction principle into elementary sulfur. This generally requires that part of the hydrogen sulfide be oxidized into sulfur dioxide in order to generate a suitable reaction gas for the Claus reaction,
2H.sub.2 S+SO.sub.2 .fwdarw.3S+2H.sub.2 O
A process of this type is described for instance in HYDROCARBON PROCESSING, April 1973, p. 107.
The sulfur recovery based on the Claus reaction suffers from the drawback that the conversion into elementary sulfur is not complete; rather a tail gas is obtained which always contains sulfur compounds, in particular hydrogen sulfide and sulfur dioxide. Even though most of sulfur compounds separated from the gas stream will be converted into elementary sulfur in a sulfur recovery based on the Claus reaction, the sulfur compounds contained in the tail gas still remain at least occasionally, in a concentration prohibiting discharge into the atmosphere. Accordingly, to further purify this tail gas, many methods have been developed which, however, suffer from the drawback of high capital costs. Such methods are described, for instance, in HYDROCARBON PROCESSING, April 1973, pp. 111-116.